Given the restrictions imposed on OTA packages by FW 5.5.x, and to scratch an itch than had been bothering me for a while, here comes the MobileRead Package Installer (aka., MRPI)!

What Does It Do?:

This is a KUAL extension, that will plug into the Helper menu a single Install MR Packages button.
Notice the plural, we'll come back to that later .
This provides an alternate to the good old Update Your Kindle UI to install the various packages offered on MR in the form of update packages (.bin).
It supports the full range of devices able to run KUAL (... but can be run outside of KUAL if need be ).

This is a KUAL extension, so it's as simple as unpacking the attached .zip file to the root directory of your Kindle over USB . You should end up with an extensions folder (with an MRInstaller subfolder) and an mrpackages folder at the USB root of your device.
As mentioned earlier, it'll plug into the Helper menu.

USAGE:

Spoiler:

It's fairly simple: upload all the update packages you need (in one go ) to the mrpackages directory, which you'll find at the root of your Kindle over USB if you installed this correctly .
Eject & unplug your Kindle, then click the Install MR Packages button in the KUALHelper menu.

Once the installer has done its job, and the UI has been brought up again, while your device should be usable straight away, some hacks may need a full restart to 'take' .

FAQs:

Spoiler:

Q. What happens if I upload an update package not made for my device?

A. Just like the OTA updater, a package not targeting your device will be skipped & destroyed. In the same vein, any package not made by our tools will be destroyed (as will non-OTA updates). This means that if you mistakenly put an official update in there, you don't have to worry about it, it'll go poof .

Q. I don't see any progress information for some packages!

A. That's the downside. Packages will need to provide their own means of user-feedback if they wish to handle progress reporting. Developers can can open up any of my latest snapshots packages and look at the libotautils/libotautils5 helper scripts for some inspiration [spoiler: there's no API, you basically do what you want ^^].